The general case

Thinking of the actual postcondition of the problem, the property could be something like:

Given any non empty collection of integers
And exist MIN and MAX that belong to the collection
Where MAX is the maximum and MIN is the minimum
Then the result is a string like "{MAX} {MIN}"

Considering for a moment the domain (all the possible instances) of any non empty collection of integers. That would include sets of a single element and also sorted elements, thus we cover the other two scenarios.

I want to generate all the integers but at the same time have the max and min so I don't write the test duplicating the implementation to find maximum and minimum.

To do that, I will generate first the min and max and then add integers in between.